This invention relates to portable label printing and applying machines.
Precise printing cannot always be obtained with such a machine because the printing pressure of the printing head against the label varies with the strength of squeezing of the operating hand lever of the machine. Even when the printing pressure is held constant, precise printing cannot be obtained because the quantity of printing ink that permeates into the label varies when the hand lever is squeezed for a long time or is released in a moment. Therefore, the quantity of ink applied to the label surface always lacks uniformity, which results in indistinct and unsatisfactory printing on labels.
For example, in the portable label printing and applying machine disclosed in Japanese Patent Publication No. Sho. 40-12335 (1965), a hand lever is pivotally secured to the grip that is integrally formed on the rear part of the machine frame. The hand lever is provided with a yoke, and a printing head is firmly attached to the front end of the yoke, without using any shock absorbing device. When the hand lever is operated, the interlocked printing head is vertically moved together with the yoke. With such a structure, neither the printing pressure nor the duration of a printing is constant.
In the portable label printing and applying machine disclosed in Japanese Patent Publication No. Sho. 47-12506 (1972), a hand lever is pivotally secured to the grip that is integrally formed on the lower part of the machine frame. The upper extension of the hand lever forms a cam to which an ink roller is rotatably attached, and a printing head is fixed to an arm that is pivotally attached to the machine frame. The printing head is urged toward a label supporting platen by means of the tension of a spring. When the hand lever is squeezed, the printing head support arm is pushed up by the cam of the hand lever. At the same time, the type faces of the printing head have ink applied to them by the ink roller. When the hand lever is then released, the printing head is automatically lowered by spring action to print labels on the platen. With this mechanism, regardless of the strength with which the hand lever is squeezed, the labels are printed with a constant printing pressure by the action of the spring. However, the type faces of the printing head remain in contact with the label after the printing stroke, so that the quantity of ink transferred to a label varies according to the duration of such contact.
As described above, the performances of known label printing and applying machines ar liable to vary during every operation of the hand lever so that the clarity of each printed label is different. Such irregularly printed labels have been acceptable because they only have been read by the naked eyes of the customers and the cashiers totaling the sales. Recently, the figures, symbols (bar codes, OCR letters) printed on labels are being read by computerized optical readers which are part of POS (point of sales) systems. In POS systems, information on stocks, sales, kinds of customers for various commodities, profits, and other working data are scanned, memorized and processed by electronic computers. Therefore, it is necessary to print labels always with clear, machine readable letters and marks.
Examples of mechanisms which provide constant printing pressure and printing time are shown in U.S. applications Ser. Nos. 720,225, filed Sept. 2, 1976, U.S. Pat. No. 4,057,452 and 723,556, filed Sept. 15, 1976, U.S. Pat. No. 4,144,810. In both of these, it is the label supporting platen that is moved. However, there are situations where it is the printing head that is to be moved.